This commit adds support for negative Fortran array strides in one
limited case, that is the case of a single element array with a
negative array stride.

The changes in this commit will be required in order for more general
negative array stride support to work correctly, however, right now
other problems in GDB prevent negative array strides from working in
the general case.

The reason negative array strides don't currently work in the general
case is that when dealing with such arrays, the base address for the
objects data is actually the highest addressed element, subsequent
elements are then accessed with a negative offset from that address,
and GDB is not currently happy with this configuration.

The changes here can be summarised as, stop treating signed values as
unsigned, specifically, the array stride, and offsets calculated using
the array stride.